Heteroporous form tool for manufacturing casting moulds and process for its manufacture

ABSTRACT

A gas-permeable form tool for manufacturing casting and core moulds from hardenable moulding sand includes a heteroporous, open-pore material. The wall of the tool contains a first fine-pore layer region adjacent to the moulding sand with a thickness of about 0.2-2 mm and a material density of about 75% to 95% of theoretical specific density and a pore diameter of about 50 μm. The first fine-pore layer comes in contact with a second, large-pore supporting skeleton having a theoretical material density of less than 80% of theoretical specific density and a median pore diameter of more than 100 μm.

The invention relates to a gas-permeable form tool for manufacturingcasting and core moulds from hardenable moulding sand and a process forits manufacture and an expedient application of such tool.

Casting moulds from moulding sand find wide-spread utilization in themanufacture of metal mould-mass produced parts. In the manufacture, onlysingle-use, expendable massive or dish-shaped moulds are used. Tomanufacture the casting mould a fine-grain moulding sand is providedwith hardenable bonding additives, conveyed over a sand intake to a formtool and hardened there. Hardening is performed thermally--at highenergy costs--or, recently, alternatively also by means of reactiongases which are pressed under pressure through the mould sand in theform tool. In accordance with the latter variant, gas is compressed intothe sand at the sand intake and must exit from the form tool throughbores, jets or other channels and openings mechanically applied to thewall of the form tool.

Pursuant to an execution known in the art (DE 24 03 199, DE 30 39 394),bores in the wall of the form tool are sealed on the outside of themould by high-pressure valves. Such form tools are disadvantageousbecause of their high tooling costs. The valves frequently becomeplugged up by grains of moulding sand entrained by gas and must becleaned. Above all, however, the wall of the form tool displays nohomogeneous gas permeability characteristics with the result that thereaction gases are unable to homogeneously permeate the moulding sand,and as a result, the moulding sand does not harden uniformly. Coremoulds can only be manufactured on a massive scale.

DE 30 02 939 describes a form tool having a wall into which ribs andslits of varying dimensions have been mechanically placed. Reaction gasentering the moulding sand through an intake is sucked off through theslits.

The slits, however, fill up with sand. Moreover, manufacturing is quiteexpensive and does not permit fabrication of a truly fine-meshed networkof slits and bores. In this execution of a form tool, too, sand is onlyunevenly permeated by the reaction gas. Additionally, excessive amountsof reaction gas are consumed, that is, in far greater amounts than arerequired according to the stoichiometry of the desired reaction.

Calls have already been made for the manufacturing of the form tool fromporous and gas-permeable materials.

Transformation of this request into reality has heretofore beenunsuccessful owing to the technical difficulties, which were to beexpected, inherent in transforming complex geometries of casting mouldsin a form tool made of porous materials as well as the requirement ofensuring homogenous gas-permeability of the wall in the micro-range aswell as its mechanical stability and of assuring, at the same time, thatthe moulding sand, when pressure charged with reaction gas, does notblock the form tool pores or even exit through the pores of the formtool wand.

The purpose of the present invention is the production of a form toolhaving a homogenous gas-permeable wall even in micro areas. Theaforementioned known methods and techniques are not suitable for doingthis. More specifically, the purpose of the invention is to produce aheteroporous form tool by combining known techniques for the productionof porous materials so that the form tool has adequate microporosity inthe first region adjacent to the form and has a large porous supportingskeleton in the region adjacent to the first region. A form tool of thissort is suited to the production of casting moulds of forms in greatnumbers, particularly the production of non-massive casting moulds inthe forms of shells. In accordance with this purpose, the surface of theform tool which comes in contact with the form sand has to be especiallywear-resistant The blocking of pores by this form sand will no longercause the form tool to fail. Pores which become blocked by form sand cannow be regenerated, easily clearing and reopening the pores.

The task of devising a gas-permeable form tool is resolved by theinvention in that the tool consists of heteroporous, open-pore material,whereby the wall of the form tool exhibits an initial fine-pore layerregion adjacent to the moulding sand 0.2-2 mm thick, having a materialdensity between about 75 to 95% of the theoretical specific density andpore diameter <50 μm and which comes in direct contact with a second,large-pore supporting skeleton having a material density less than about80% of the theoretical specific density, and a median pore diameter >100μm. Executions according to other aspects of this invention 10 haveshown themselves to be particularly expedient for the gas-permeable formtool and process for its manufacture and advantageous application.

Casting and core moulds, that is, moulds for the production of massiveas well as internally hollow casting parts form a part of form tools.

To achieve the required material and structural characteristics of theform tool wall according to the present invention, the usual technicalexpert has a number of individual processes at his disposal for themanufacture of porous tools, which processes should be advantageouslycombined.

Metallic and/or ceramic materials and/or plastics are the basicmaterials used for the form tool wall. In a single form tool accordingto designs known in the art, up to 60,000 sand moulds are manufacturedaccording to size. For reasons of economy, the sand is poured into themould at high speed and under high pressure. Wear and tear to thesurface of the form tool coming in contact with moulding sand iscommensurately high. This situation must be taken into consideration inthe choice of the material of the fine-pored layer of the form tool.Wear-resistant types of steel as well as wear-resistant ceramics andmetallic and non-metallic hard materials, e.g., silicon nitride, boronnitride, titanium carbide, titanium nitride, silicon carbide, haveproved useful for this layer.

The heteroporous wall of the form tool can be formed from eitherviscous, effervesced and subsequently solidified material, or the wallis moulded by means of powdery raw materials subsequently solidified.

The layer of the wall of the form tool which comes into contact with themoulding sand can be formed by compressing powder isostatically on agauge mould according to the casting part. The powder can, having beenmixed with a volatile solvent, be applied as a paste to the gauge mouldor sprayed thereon. Galvanic processes and gas precipitation processes(PVD processes) have shown themselves to be useful in forming saidlayers. Finally, the layer can be applied to the gauge mould in the formof a flexible metallic or ceramic film, foil or thin sheet ("films").The flexibility of such films is provided by extremely flexiblethermoplastic components which, when solid, evaporate during subsequentheat treatment Films can also, moreover, consists of powdery metals,hard materials or ceramics.

The gauge mould to which the layer material has been applied isthereupon either foamed up or, following embedding in a correspondingouter mould, filled with coarse-grained powdery material and,preferably, isostatically compressed.

The finished compound body is produced by thermal or chemical hardening,burning or sintering of the compacted compound materials.

In the manufacture of the open-pore supporting skeleton, it has shownitself to be expedient to coat sand, glass or ceramic grains first witha thin plastic coating through dipping in corresponding dispersions orsolutions.

Granulate so pretreated can be poured and/or compressed into a mould andsubsequently chemically or thermally hardened.

Techniques to obtain fine-pore or large-pore and open-pore materials areknown in the art. Thus, for example, in the manufacture of diaphragmsfor electrodes in electro-chemistry techniques utilizing special poreforming agents have been developed which produce a material structure ofa defined gas-permeability of the kind required in the present case.Techniques for the manufacture of coarse- and open-pore materials havebeen developed in the broad applications field of mechanical filters as,for example, in the field of self-lubricating friction bearings orelectrical contact materials consisting of porous skeleton of a materialA into which material B is infiltrated.

Form tools in accordance with the present invention display amultiplicity of advantages. They exhibit an open-pore wall constructionwith a defined drop in pressure which is completely homogeneous throughto the micro-region. The pressure drop enables uniform gas permeationthrough the wall and, consequently, homogeneous hardening of themoulding sand. The pores in the fine-pore region of the wall of the formtool are composed in such a way that only in extraordinary circumstancesare grains of sand able to accumulate in the wall of the form tool. Ofdecisive importance, however, is the fact that these grains of sand can,as a rule, be removed again from the pores with little effort by blowingair under high pressure, possibly together with solvent damping down,from the direction of the coarse-pore skeleton of the wall of the formtool through the fine-pore wall layer.

In contrast to processes known in the art whereby gas hardening of themoulding sand is accomplished by blowing in gas via the sand intake,pressurization of the moulding sand confined in the form tool can, whereform tools according to the invention are used, be accomplished throughthe heteroporous wall. By correspondingly regulating gas pressure andtime it is possible to effect hardening of the confined moulding sandonly in a marginal area up to a desired depth. More precise dosaging canbe achieved by saturating the form tool with a suitable liquid. By sodoing, a specific capillary pressure is created in the fine pores of thewall of the tools which releases the reaction gas only when thispressure is exceeded. The core of the confined sand, given correspondingstoichiometric dosage of the gas, remains friable and, followinghardening of the marginal area, can be removed via the sand intake andrecycled.

An intrinsic advantage of form tools according to the present inventionlies in the possibility of adapting their surface facing the mouldingsand to the desired casting mould and of configuring their rear surface,however, with few plane surfaces, e.g., square-shaped or cylindrical.Owing to the gas-induced pressurization of the moulding sand through theporous wall of the form tool a fine layer of gas regularly forms betweenthe wall of the form tool and the moulding sand, thereby precludingadherence of the moulding sand to the form tools wall during the sandhardening process. The sand mould is easily detached from the form toolfollowing the hardening process. Special measures to prevent theadherence of moulding sand and the form tool (spraying of the form toolwall, insertion of a film), as are required with tools and processes forthe manufacture of casting moulds known in the art, can, as a rule, beomitted. The technique of subsequent applications of a fine-pore layerand skeleton materials to the gauge mould makes it possible to give theform tool directly its definitive shape, surface characteristics andwear-resistance qualities. Consequently, neither a cost-intensivemechanical finishing of the surface of the wall of the form tool toproduce the desired geometry and surface roughness is necessary nor isre-treatment, in particular, thermal hardening processes, to achieverequisite surface hardness and resistance to wear--in contrast toprocesses heretofore utilized in manufacturing form tools which do notstart off with porous materials.

The invention is described in more detail by means of FIG. 1 as well astwo practical embodiments.

FIG. 1 shows the shape of a half-shell of a form tool, in a sectionalview, as well as devices for the manufacture of the form tool accordingto a preferred process. The sectional view provided by FIG. 1 shows, inparticular, the match plate 1 together with the gauge mould for thehalf-shell of a form tool. In the sectional view that region of thematch plate is particularly marked which, during later use, provides thesand intake of the form tool 1a. A sealing plate 2 bears on the matchplate or is screwed or clamped thereto. It has a central recesscorresponding to the geometric form of the form tool to be produced. Thefine-pore layer region 3 of the form tool adjacent to the moulding sanddisplays a constant layer thickness over the entire surface area, exceptfor a narrow region at the dividing surface of both half-shells. Theopen pore supporting skeleton 4 is in direct contact with the fine-porelayer region of the form tool. The outer geometric shape of the formtool is provided by a moulding box 5 or frame which has been screwedonto the match plate. Manufacturing variants are possible in thisconfiguration whereby the moulding box is not completely filled up withthe material but, rather, whereby, during the filling up with a materialwhich can flow or be coated, an air space 6 remains between thesupporting skeleton and the top of the moulding box.

EXAMPLE 1

According to the technique shown in FIG. 1 (for the manufacture of theform tool), first a match plate with the gauge mould of a half of thecasting part to be manufactured is produced according to standardprocedures from a metallic and/or ceramic material or plastic. In themajority of cases it is best, with core and casting moulds, tomanufacture the form tool from two half-shells. After previouslyapplying a separating compound, a sealing plate, preferably made ofsteel or ceramic, is applied to the match plate and connected by screwsto the match plate. The central recess in the sealing plate isdimensioned in such a way that in the region of the dividing surface ofboth half-shells of the form tool between the gauge surface (matchplate) and sealing plate, a clearance of the thickness of the fine-porelayer region of the form tool remains.

The fine-pore layer of the form tool is first applied to the gaugesurface of the match plate--if necessary, following prior application ofa separating compound to the gauge surface. A paste is brushed on orsprayed on for this purpose. The paste consists of fine-grained,corrosion-resistant ceramic powder whose grain size is, on the average,10--100 μm thick, to which powder, in order to increase form toolsurface wear resistance, 10-20% volume of titanium carbide powder(measured proportionate that amount of ceramic powder) having theapproximate same size of grains is added. The powder is worked up to apaste using a volatile, thermally evaporable bonding agent. To thebonding agent, where necessary, non-volatizing metallic and/ornon-metallic components and/or pore forming agents have been added.Application of the fine-pore layer is preferably performed in severallayers until the desired overal layer thickness has been achieved In theprocess, the layer application according to FIG. 1 also is performedbeyond the edge of the sealing plate.

The fine-pore layer applied in this manner is dried or hardened.Subsequent thereto, a moulding box or moulding frame as per FIG. 1 isscrewed onto the match plate or sealing plate and the material used toform the wall area with the open-pore skeleton is placed into themoulding box. For this purpose, a coarse-grain ceramic powder to whichvolatile pore forming materials have been added, is used. Such materialsare of the type used to manufacture porous ceramic filters, for example.The ceramic powder is stirred together with volatile bonding agents toform a paste which is coated on to the moulding box and allowed toharden thereon. Thereafter the form tool is separated from the matchplate and sintered or burned in high-temperature ovens. In this mannerone obtains wear-resistant, ready-to-assemble form tool half-shells withplane separating surfaces. The mould surface, as a rule, does notrequire any surface refinishing. The area of the sand intake is finallysealed off with a pore filler so that during future operation noreaction gas can penetrate through this area of the form tool wall andso that the moulding sand cannot harden in this area.

Inspection of form tools manufactured in this way with the wallconstruction according to the invention has shown that a 1-2 bardifferential pressure can be created at the boundary between the coarse-and fine-layer. The resulting range of variation of the absolute gaspressure before the boundary in the coarse-pore section of the wall invarying sections of the wall of the form tool or in various form toolsmanufactured according to the same process lies between 0.1-0.2 bar andis, therefore, to a great extent independent of the actual thickness ofthe coarse-pore supporting skeleton of the wall of the form tool. Theaforementioned jump in the gas pressure at the boundary between thecoarse- and fine-pore layer occurs practically speaking solely onaccount of the structure of the fine-pore layer. This pressure jump canbe maintained further by saturating the form tool with a suitablesealing liquid whereby a very homogeneous capillary pressure is formedover the entire surface area of the form tool in the pores of thefine-pore layer.

The manufacture of a casting mould from moulding sand using a form toolaccording to the present invention proceeds as follows. After havingbeen filled with moulding sand, the form tool is charged externally withreaction gas having a pressure of >2 bar. This gas forces the liquid outof the capillaries of the fine-pore layer of the form tool and, at a gaspressure which can be precisely regulated, reaches the moulding sand ora marginal area of the sand mould That enables the moulding sand to behardened to the desired, easily regulatable depth. The core region ofthe moulding sand poured in continues to be friable and, followingconclusion of the hardening process, can be removed via the sand intakeand recycled. When gas pressure falls below 2 bar, the sealing liquid,by the wick effect, is drawn back again into the pores of the fine-porelayer. That means short fabrication times for the individual sand mouldsas well as low susceptibility to trouble and low scrap levels.

EXAMPLE 2

Analogous to Example 1, a gauge mould or match plate for a form tool ismanufactured. Also as in Example 1 a sealing plate is clamped onto thematch plate. The form tool wall material for the fine-pore layer isapplied to the gauge mould in the form of a flexible metallic film Theseparately fabricated metallic film consists of a homogeneous mixture ofcorrosion-resistant steel particles with a grain size distributionvarying from 10-100 μm, where necessary, enriched with some percent pervolume of wear-resistant titanium carbide particles of comparable grainsize, where necessary, supplemented by powdery fillers and pore-formingmaterials as well as of a thermoplastic plastic which is volatilized athigher temperatures. By means of techniques known in the art forisostatic powder-tube compression, a rubber or plastic "tube" isthereupon clamped onto the base of the match plate and filled with acoarse-grain powder mixture consisting of alloyed iron powder andpore-forming agents --covering over the fine-pore layer. The inside ofthe tube is thereupon evacuated, the tube sealed off. The entire unit iscold-isotatically compacted. The unfinished cast of the form tool isseparated from the match plate and further processed using standardsintering processes. The sintered form tool can--to the extentnecessary--be mechanically machined and, for example, sized for mountingin tool mounting supports.

It is customary, during the isostatic powder compaction in plastic orrubber sheaths, to give the rubber sheath the rough form or roughcontours of the form piece to be compacted. Accordingly, with respect tothe case at hand, half-shells of form tools having an approximatelyhomogeneous form tool wall thickness can be achieved.

As already mentioned hereinbefore, a multiplicity of techniques,commensurate with the broad applications filed for porous mouldedbodies, is known in the art for producing fine-pore and/or large-poremoulded bodies from powdery materials. The description for themanufacture of form tools is provided with reference to those productgroups and is not intended to be in any way definitive

I claim:
 1. A gas-permeable form tool for manufacturing casting and coremoulds from hardenable moulding sand, the form tool comprisingheteroporous, open-pore material, said form tool having a heteroporouswall including a first fine-pore layer region adjacent to a mouldingsand to be conveyed to said form tool and hardened, by blowing reactiongas towards said moulding sand through said heteroporous wall, saidfirst fine-pore layer being about 0.2-2 mm thick and having a materialdensity between about 75 to 95 percent of theoretical specific densityand a pore diameter of less than about 50 μm, said first fine-pore layerbeing in contact with a second large-pore supporting skeleton having atheoretical material density of less than 80 percent of theoreticalspecific density and a median pore diameter of more than about 100 μm,the boundary between said first fine-pore layer and said secondlarge-pore layer providing a differential pressure when said form toolis charged externally with said reaction gas.
 2. A form tool accordingto claim 1, wherein the material is an open-pore, solidified foam.
 3. Aform tool according to claim 1, wherein the tool comprises a ceramicmaterial.
 4. A form tool according to claim 1, wherein the toolcomprises a metallic material.
 5. A form tool according to claims 1, 2,3 or 4 wherein at least one of the regions comprises at least two layersof homogeneous structure and material composition.
 6. A form toolaccording to claim 5 wherein the wall regions of different porethickness comprise different materials.
 7. A form tool according toclaim 5 wherein inner surface of said tool is shaped to produce acomplex geometry of the casting mould desired and the outer surface ofsaid tool includes a plane surface.
 8. A form tool according to claim 5wherein the form tool comprises two or more parts.
 9. A process formanufacturing a form tool to be used for manufacturing a casting mould,said form tool having a heteroporous wall, comprising the followingsteps:applying a first layer of a fine-grain powdered first material toa gauge mould of said casting mould said first layer being about 0.2-2mm thick and having a material density between about 75 to 95 percent oftheoretical specific density and a pore diameter of less than about 50um; applying by compression a coarse-grain powdered second layer assupporting skeleton having a theoretical material density of less than80 percent of theoretical specific density and a median pore diameter ofmore than about 100 um, said first layer being in contact with saidsecond layer; and setting the wall of the form tool.
 10. A process formanufacturing a form tool as in claim 9 wherein a pore-forming agent isadded to at least one of the powdered materials prior to application ofthe materials.
 11. A process for manufacturing a form tool according toclaim 10 wherein setting of the form tool is accomplished by sintering.